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DIAGNOSTIC RADIOLOGY F luoroscopy

DIAGNOSTIC RADIOLOGY F luoroscopy. Chapter 3 Aim: To become familiar with the component of the fluoroscopy system (design, technical parameters that affect the fluoroscopic image quality and Quality Control). Fluoroscopy system. Different fluoroscopy systems. Remote control systems

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DIAGNOSTIC RADIOLOGY F luoroscopy

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  1. DIAGNOSTIC RADIOLOGYFluoroscopy Chapter 3 Aim: To become familiar with the component of the fluoroscopy system (design, technical parameters that affect the fluoroscopic image quality and Quality Control).

  2. Fluoroscopy system Add module code number and lesson title

  3. Different fluoroscopy systems • Remote control systems • Not requiring the presence of medical specialists inside the X-ray room • Mobile C-arms • Mostly used in surgical theatres. Add module code number and lesson title

  4. Different fluoroscopy systems • Interventional radiology systems • Requiring specific safety considerations. Interventionalists can be near the patient during the procedure. • Multipurpose fluoroscopy systems • They can be used as a remote control system or as a system to perform simple interventional procedures Add module code number and lesson title

  5. Topic 2 : Image Intensifier component and parameters

  6. Electrode E1 Input Screen Electrode E2 Electrode E3 Electrons Path Output Screen Photocathode +

  7. Image intensifier systems Add module code number and lesson title

  8. Image intensifier component • Input screen: conversion of incident X-rays into light photons (CsI) • 1 X-ray photon creates  3,000 light photons • Photocathode: conversion of light photons into electrons • only 10 to 20% of light photons are convertedinto photoelectrons • Electrodes : focalization of electrons onto the output screen • electrodes provide the electronic magnification • Output screen: conversion of accelerated electrons into light photons

  9. Image intensifier parameters (I) • Conversion coefficient (Gx): the ratio of the output screen brightness to the input screen dose rate [cd.m-2Gys-1] • Gx depends on the quality of the incident beam (IEC publication 573 recommends HVL of 7  0.2 mm Al) • Gx is directly proportional to: • the applied tube potential • the diameter ()of the input screen • input screen of 22 cm  Gx = 200 • input screen of 16 cm  Gx = 200 x (16/22)2 = 105 • input screen of 11 cm  Gx = 200 x (11/22)2 = 50

  10. Image intensifier parameters (II) • Brightness Uniformity: the input screen brightness may vary from the center of the I.I. to the periphery Uniformity = (Brightness(c) - Brightness(p)) x 100 / Brightness(c) • Geometrical distortion: all x-ray image intensifiers exhibit some degree of pincushion distortion. This is usually caused by either magnetic contamination of the image tube or the installation of the intensifier in a strong magnetic environment.

  11. Image distortion

  12. Image intensifier parameters (III) • Spatial resolution limit: the value of the highest spatial frequency that can be visually detected • it provides a sensitive measure of the state of focusing of a system • it is quoted by manufacturer • it can be measured optically • it correlates well with the high frequency limit of the Modulation Transfer Function (MTF) • it can be assessed by the Hüttner resolution pattern

  13. Line pair gauges

  14. Line pair gaugesGOOD RESOLUTIONPOOR RESOLUTION

  15. Image intensifier parameters (IV) • Overall image quality: threshold contrast-detail detection • X-ray, electrons and light scatter process in an I.I. can result in a significant loss of contrast of radiological detail. • The degree of contrast is effected by the design of the image tube and coupling optics. • Spurious sources of contrast loss are: • accumulation of dust and dirt on the various optical surfaces • reduction in the quality of the vacuum • aging process (destruction of phosphor screen) • Sources of noise are: • X-ray quantum mottle • photo-conversion processes

  16. Image intensifier parameters (V) • Overall image quality can be assessed using: A contrast-detail detectability test object (array of disc-shaped metal details which gives a range of diameters and X-ray transmission • Sources of image degradation such as contrast loss, noise and unsharpness limit the number of details that are visible. • Image quality can be detected as a reduction in the number of low contrast and/or small details.

  17. Overall image quality

  18. Topic 3 : Image Intensifier and TV system

  19. Image intensifier - TV system • Output screen image can be transferred to different optical displaying systems: • conventional TV • Generating a full frame of 525 lines (in USA) • 625 lines and 25 full frames/s up to 1000 lines (in Europe) • interlaced mode is used to prevent flickering • cinema • 35 mm film format: from 25 to 150 images/s • photography • rolled film of 105 mm: max 6 images/s • film of 100 mm x 100 mm

  20. kV X-RAY TUBE PM REFERENCE kV FILM CONTROLLER VIDICON GENERAL SCHEME OF FLUOROSCOPY Add module code number and lesson title

  21. Type of TV camera • VIDICON TV camera • improvement of contrast • improvement of signal to noise ratio • high image lag • PLUMBICON TV camera (suitable for cardiology) • lower image lag (follow up of organ motions) • higher quantum noise level • CCD TV camera (digital fluoroscopy) • digital fluoroscopy spot films are limited in resolution, since they depend on the TV camera (no better than about 2 lp/mm) for a 1000 line TV system

  22. TV camera and video signal (I) • Output phosphor of image intensifier is optically coupled to a TV. • A pair of lenses focuses the output image onto the input surface of the television camera. • Often a beam splitting mirror is used in order to reflect part of the light onto a 100 mm camera or cine camera. • Typically, the mirror will reflect 90% of the incident light and transmit 10% onto the television camera. • Older fluoroscopy equipment have a television system using a camera tube with a conductive layer. • In a PLUMBICON tube, this layer is made of lead oxide, whereas in a VIDICON, antimony trisulphide is used

  23. Deviation coil Steering coils Alignement coil Photoconductive layer Accelarator grids Focussing optical lens Input plate Control grid Electron beam Iris Video Signal Electron gun Signal electrode Field grid Electrode Photoconductive camera tube

  24. TV camera and video signal (III) • The surface of the photoconductor is scanned with an electron beam and the amount of current flowing is related to the amount of light. • The scanning electron beam is produced by a heated photocathode. • Electrons are emitted into the vacuum and accelerated across television camera tube by applying a voltage. • Electron beam is focussed by a set of focussing coils.

  25. TV camera and video signal (IV) • This scanning electron beam moves across the surface of the TV camera tube in a series of lines by a series of external coils. • In a typical television system, on the first pass the set of odd numbered lines are scanned followed by the even numbers (interlaced). • The purpose of interlacing is to prevent flickering of the television image on the monitor, by increasing the apparent frequency of frames (50 half frames/second). • In Europe, 25 frames are updated every second.

  26. Different types of scanning 11 1 INTERLACED SCANNING 13 12 3 2 15 14 625 lines in 40 ms i.e. : 25 frames/s 5 4 17 16 7 6 19 18 9 8 21 20 10 1 2 3 4 5 6 7 PROGRESSIVE SCANNING 8 9 10 11 12 13 14 15 16 17 18

  27. TV camera and video signal (V) • The video signal comprises a set of repetitive synchronizing pulses. In between there is a signal that is produced by the light falling on the camera surface. • The synchronizing voltage is used to trigger the TV system to begin sweeping across a raster line. • Another voltage pulse is used to trigger the system to start rescanning the television field. • A series of electronic circuits move the scanning beams of the TV camera and monitor in synchronism. • The current, which flows down the scanning beam in the TV monitor, is related to that in the TV camera. • Consequently, the brightness of the image on the TV monitor is proportional to the amount of light falling on the corresponding position on the TV camera.

  28. TV camera and video signal (VI) • On most fluoroscopy units, the resolution of the system is governed by the number of lines of the television system. • Thus, it is possible to improve the high contrast resolution by increasing the number of television lines. • Some systems have 1,000 lines and prototype systems with 2,000 lines are being developed.

  29. TV camera and video signal (CCD) • Many modern fluoroscopy systems used CCD (charge coupled devices) TV cameras. • The front surface is a mosaic of detectors from which a signal is derived.

  30. Schematic structure of a charged couple device (CCD)

  31. TV image sampling IMAGE 512 x 512 PIXELS HIGHT 512 ONE LINE VIDEO SIGNAL (1 LINE) 64 µs SYNCHRO 12 µs DIGITIZED SIGNAL SAMPLING LIGHT INTENSITY

  32. Digital radiography principle ANALOGUE SIGNAL I t ADC Memory DIGITAL SIGNAL Iris Clock t

  33. Where to Get More Information • Physics of diagnostic radiology, Curry et al, Lea & Febiger, 1990 • Imaging systems in medical diagnostics, Krestel ed., Siemens, 1990 • The physics of diagnostic imaging, Dowsett et al, Chapman&Hall, 1998 Add module code number and lesson title

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